Metal delivery system for nanoparticle manufacture

ABSTRACT

Described is a method for the production of pure or mixed metal oxides, wherein at least one metal precursor that is a metal carboxylate with a mean carbon value per carboxylate group of at least 3, e.g. the 2-ethyl hexanoic acid salt, is formed into droplets and e.g. flame oxidized. The method is performed at viscosities prior to droplet formation of usually less than 40 mPa s, obtained by heating and/or addition of one or more low viscosity solvents with adequately high enthalpy.

TECHNICAL FIELD

The present invention concerns a specific type of manufacturing methodfor metal oxides and metal oxides with specific features that areobtainable by said method, in particular cerium, zirconium, gadolinium,iron, manganese oxides, mixed oxides, in particular earth alkalinetitanates, alkali manganates, stabilized zirconia and ceria.

BACKGROUND ART

Metal oxides, in particular mixed metal oxides have a broad range ofapplications such as e.g. ceramics, polymer additives, fillers,pigments, reactive surfaces, catalysts, storage materials, polishingadditives, membranes, fuel cells etc. Among the most important metaloxides are cerium oxide, cerium-zirconium and other stabilized zirconiamixed oxides, titanates and other mixed oxides below referred to asceramic oxides. If these materials are used as nanoparticles (particlediameter below 200 nm), they exhibit advantageous properties such ashigh catalytic activity, improved processing capability, low sinteringtemperature, good dispersion capability, just to name a few. Titanatesare used as dielectics for capacitors. Nanoparticles are of highinterest as reduced feature size requires thinner sheets of dialecticsand since such thinner sheets are preferable made from very smallparticles, for example, nanoparticles.

Current methods for the production of metal oxides are mechanical andmechanical/thermal processes, wet-phase chemistry based methods, andhigh temperature methods such as flame spray pyrolysis (FSP). For thelatter, energy to drive the conversion to nanosized oxides can beradiofrequency (plasma), microwaves, laser or shock waves. Mostconvenient, however, is the use of thermal energy since in most casesthis is the least expensive source of energy.

Mechanical and mechanical/thermal methods are energy intensive(milling!) and generally suffer from insufficient mixing at the atomiclevel leading to low phase stability and/or low specific surface area.Impurity from the milling (abrasion) reduces product purity andperformance.

Wet-phase based methods entail huge solvent costs, produce large amountsof waste water and need calcination steps after the synthesis, makingthem cost intensive. Furthermore, although e.g co-precipitation ofceria/zirconia can lead to mixed oxide powders with extremely highspecific surface areas, unfortunately, the temperature stability ofas-prepared oxides is characterized by a big loss of specific surfacearea at elevated temperature. The same observation applies for mostwet-phase made ceramics. Preparation at high temperature may produce anoxide with increased stability. This has prompted several people toattempt to prepare oxides by flame spray based methods. Flame spraypyrolysis (FSP) is a known process and has been used for preparation ofmany oxides. It uses thermal energy and has the inherent advantage ofsupplying low cost energy to drive nanoparticle formation. However, inthe case of many oxides, the research for suitable precursors entailshuge problems associated with the chemical properties of thesecompounds. For example Yoshioka et al. (1992) used FSP for theproduction of ceria oxides, but they received a powder of low specificsurface area. WO 01/36332 discloses a FSP method leading to aninhomogeneous product comprising ceria particles of broadly varyingsizes. Aruna et al. (1998) investigated the ceria/zirconia synthesis bycombusting mixtures of redox compounds and oxidizing metal precursors.This high temperature preparation yielded a high surface area productwith apparently good phase mixing in as-prepared powders. However, thepreparation of ceramics by solid combustion is difficult to realize athigh production rates, since the process may quickly run out of control.Furthermore it is basically a batch process and the reproducibility is ageneral problem. Laine et al. (1999) and Laine et al. (2000) used aspray pyrolysis unit to prepare ceramic oxides but the specific surfacearea of the product powder stayed low, at 10 to 16 m²/g. EP 1 142 830also discloses a FSP method for the preparation of ceria/zirconiastarting from organometallic compounds in organic solvents and/or water.The procedure disclosed in EP 1 142 830 focuses on chlorine free powdersproduced by flame spray pyrolysis and uses precursor solutions of typeMeR where R is an organic rest such as methyl, ethyl, or a correspondingalkoxy-rest or a nitrate anion. As solvents, water or alcohols are used.U.S. Pat. No. 5,997,956 discloses a procedure where a liquid or liquidlike fluid near its supercritical temperature is injected in a flame orplasma torch and thereby converted to nanoparticles.

WO 02/061163 A2 discloses an apparatus for the production of powders orfilm coatings. Thereby, the metal containing liquid is atomized withoutthe use of a dispersion gas. Oljaca et al. (2002) describe a processusing similar nozzles for the manufacture of nanoparticles. They onlydescribe very low production rates with solutions being less than 0.05 Min metal. Droplet size distribution is stated as a major parameter forthe successful nanoparticle synthesis. They report on the synthesis ofyttria stabilized zirconia amongst others.

Recently Mädler et al. (2002B) disclosed an FSP method for theproduction of pure ceria with high surface and homogeneous particlesizes using a two phase nozzle to disperse the metal containing liquidby a dispersion gas (oxygen or air) and igniting the resulting spray bya premixed falame surrounding said nozzle. Such burner is furtheron inthis document termed a spray burner. The solvent system used by Mädleret al., however, has now been found to be unsuitable for the productionof e.g. ceria/zirconia. Stark et al. (2003) disclose the use of aceticacid and lauric acid for the preparation of ceria, zirconia andceria/zirconia. Maric et al. (2003) use a not further specifiedCxH2zCeO6 precursor for the preparation of ceria, gadolinia and samariadoped ceria for fuel cell membranes. They applied a dispersion gas freeatomization device working at low production rate and using a Nanomiserdevice (WO 02/061163 A2, see above) that makes very small droplets(below 10 micrometer). Overall production rate even using such amultiple nozzle setup is still below 1 kg/h.

In order to bring the nanoparticle manufacture from the pilot-scaleproduction to an industrial scale synthesis (kg to ton quantities), someadditional problems are to be faced. The most prominent is the choice ofreadily accessible metal precursors that allow sufficiently highproduction rates. The present invention links the manufacture ofnanoparticles to existing metal containing products that were developedfor different applications but not the manufacturing of nanoparticles. Asecond problem is production rate. Using multiple arrays as in WO02/061163 A2 entails problems with maintenance, nozzle clogging, space,reproducibility and others. It would be much preferred to use fewburners to make the same quantity of powder. It would further be of muchuse to apply a metal carrier liquid that can be sprayed on mostconventional oil burners and does not require the sophisticatedatomisation devices as e.g. in WO 02/061163. This further much helpsscaling the production further up as oil burners with well above 100 kgoil/h are available. As it will become apparent within this invention,such a burner could achieve up to 20 kg ceramic particles per h (for 100kg feed/hour).

For e.g. ceria, zirconia and ceria/zirconia all hitherto known methodsuse dilute metal solutions (usually <0.15 moles of metal/liter)resulting in low production rates. High metal concentrations arefavorable as they directly increase the production rate of the process.Therefore, the metal concentration in the carrier liquid should be ashigh as possible. In the scope of the present invention, the flame sprayprocess was found to limit the range of possible carrier liquidformulations by the viscosity as the liquid has to be dispersed duringthe process. While droplet size was found to be of minor importance,very viscous liquids could not be sprayed at all. It is therefore ofhigh interest to find precursors for flame spray synthesis of oxide andmetal nanoparticles that combine low viscosity and high metalconcentration. Furthermore, such formulations should be readily producedand be stable upon storage. It is yet another objective of the presentinvention to show that common oil burners can be used for the synthesisof nanoparticles if the metal carrier liquid exhibits the abovementioned characteristics.

DISCLOSURE OF INVENTION

Hence, it is a general object of the invention to provide a methodsuitable for the production of metal oxides with improved features andtherefore extended applications as well as such metal oxides.

Another object of the present invention is a ceria, zirconia, stabilizedzirconia, iron or manganese oxide, lithium manganate or calcium- andbarium-titanate nanopowder with high homogeneity and produced at highproduction rate.

Still other objects of the present invention are the use of a metaloxide of the present invention as at least part of a catalyticallyactive system, in particular for combustion engines, or formechanochemical polishing, or in magnets, in electronic components,mechanical actuators, as piezoelectric or energy storage elements.

It is yet another objective of the present invention to stabilizezirconia by adding another metal oxide such as ceria, gadolinia oryttria for applications in fuel cells, sensors and as structural ceramicor for coatings.

It is again another objective to show that common oil burners can beused for the synthesis of nanoparticles if metal containing liquids ofspecific quality are applied.

It is another objective that technical metal formulations are wellsuited for nanoparticle synthesis. While impurities are reducing thethermal stability, such particles may still be of high interest forhigh-volume applications where purity is of minor importance. Suchapplications may be as opacifiers in ceramics (low grade zirconia).

Now, in order to implement these and still further objects of theinvention, which will become more readily apparent as the descriptionproceeds, the metal oxides of the present invention are manifested bythe features that they are obtainable by the method of the presentinvention.

The method for the production of a metal oxide of the present invention,is characterized in that at least one metal carboxylate (the salt of ametal with a carboxylic acid) is dissolved in a high enthalpy(usually >25 kJ/g) solvent comprising up to at most 40% of a carboxylicacid or a carboxylic acid mixture to form a solution, and wherein saidsolution is then formed into droplets and converted to nanoparticle bymeans of a high temperature process. Such metal carboxylates, also knownas metal soaps, are generally used in large quantity as siccatives inresins, lacquers, as additives in polymer manufacturing, as fueladditives and in the fabrication of thin films. Some also find use asmetal source for animal skin preservation. Therefore, such metal soapsare readily available, stable and readily processed, or they can beobtained by treating metal precursors with a suitable acid, obtionallyand often preferably at higher temperature. Suitable metal precursorsare those with anions that are easily replaced. Preferred anions arethose that in protonated form are easily removable, such that noequilibrium can establish and the reaction proceeds to almost 100%, e.g.oxides, hydroxides, carbonates, halides, such as chlorides and bromides,and lower alkoxides.

Either pure (many of these metal soaps have a low melting point suchthat at enhanced temperature they have a sufficiently low viscosity) ordissolved in an additional solvent, such metal soaps are a highlysuitable precursor for the manufacture of nanoparticles. Part of theirmost advantageous use is the fact that very high production rates can beused (in dissolved form, more than 100 g of ceria per liter of carrierliquid, or over 200 g zirconia per liter carrier liquid) which becomes amajor problem in conventional preparation methods. There, solubility ofthe metal or high viscosity (difficult to spray) limit the productionrates to low values (Mädler et al., 2002 achieved 26 g ceria per literof precursor liquid). Oljaca et al. (2002) used less than 0.05 Msolutions (corresponds to 8.6 g ceria or 6 g zirconia/h) while accordingto the invention above 1 M solutions can be made.

It has been found that using a precursor mix as disclosed in the scopeof the present invention in FSP allows the production of pure and mixedoxides such as ceria, zirconia, gadolinia, titanates, manganates andstabilized zirconia at high production rate while preserving thebeneficial properties of good mixing at atomic level, excellent specificsurface area (e.g. good accessibility) and high phase stability.

A precursor or precursor mix, respectively, for FSP needs to carrysufficient metal(s) into a high temperature zone or preferably theflame, distribute said metal(s) within the high temperature zone or theflame and support a stable combustion or conversion and spray process.In the case of many metals such as for example cerium, this entails thefollowing problems:

-   -   Few organometallic compounds are known, all organometallic        compounds are rather expensive and/or contain other, undesired        elements such as halogenes.    -   Cheap precursors are mainly water soluble. Water, however, is a        very bad basis for FSP since it is cost and equipment intensive        to achieve the necessary high temperature (plasma, laser,        micorwaves, high dilutions).

The here disclosed process avoids these limitations.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and objects other than those setforth above will become apparent when consideration is given to thefollowing detailed description thereof. Such description makes referenceto the annexed drawings, wherein:

FIG. 1 shows a conventional oil burner (dispersion nozzle) with guidingtube removed top view. The middle consists of a 2-phase nozzle where theoil or, in this case, metal containing liquid is dispersed. Thesectionned channel delivers air to support the combustion.

FIG. 2 shows a conventional oil burner producing calcia stabilizedzirconia nanoparticles at 540 g/h.

FIG. 3 shows transmission electron microscopy images of ceria/zirconia(CeZrO4) prepared from a 0.4 M solution of Ce and Zr (each 0.4 M). Onthe left is a transmission electron microscope (TEM) picture ofceria/zirconia Ce_(0.5)Zr_(0.5)O₂ as prepared by using a production rateof 118 g/l of carrier liquid. On the right is a transmission electronmicroscope (TEM) picture of ceria/zirconia Ce_(0.5)Zr_(0.5)O₂ afterthermal treatment at 700° C., 16 h, air, showing that such thermaltreatment leads to larger particles of similar shape.

FIG. 4 shows an X-ray diffraction spectrum (XRD) diagram of CeZrO4prepared from 0.4 M solutions of Ce and Zr.

FIG. 5 shows the X-ray diffraction spectrum (XRD) of gadolinia dopedceria (Gd0.1Ce0.9O1.95; gadolinia/ceria) as prepared. It depicts thebroad signals of very small crystals and confirms the excellent degreeof mixing of the two metals and—by the clear peaks—it underlines thephase stability of the mixed oxide. It shows how different oxides can bedelivered in the flame extremely homogeneously as to form a pure mixedoxide phase.

FIG. 6 depicts the XRD diagram of manganese oxide nanoparticles preparedfrom manganese naphthenate.

FIG. 7 is a XRD diagram of BaTiO3 prepared from barium and titaniumprecursors, dissolved in 2-ethylhexanoic acid, and with xylene assolvent. It confirms the identity of barium titanate nanoparticles byXRD.

FIG. 8 is a XRD diagram of calcium titanate from calcium octoate andtitanium octoate prepared from titanium tetra isopropoxide and2-ethylhexanoic acid showing that calcium titanate can be obtained fromcalcium and titanium containing precursors that are converted into metalsoaps. The low curve gives the XRD as prepared and the top gives theoxide after a sintering at 600° C., 2 h in air. A TEM image of theproduct is shown in FIG. 9.

FIG. 9 gives TEM images of electronic materials prepared as in theexperimental procedure. Left: LiMn2O4 particles for usage in batterystorage materials, right: CaTiO3 for usage as dielectric or in otherapplications. In both materials, rather uniform particle size isobserved, indicating a homogeneous particle formation throughout thereactor.

FIG. 10. is the XRD diagram of gadolinium oxide prepared from thecorresponding gadolinium octoate in xylene/2-ethylhexanoic acid (10:1 byvolume).

FIG. 11 gives the XRD diagram of lithium manganese oxide. The asprepared powder mainly consists of hausmannite and some amorphous parts.Heating to 400° C. is sufficient for the formation of the mixed oxidespinell phase.

MODES FOR CARRYING OUT THE INVENTION

The metal oxides of the present invention are obtainable by a methodwherein at least one metal carboxylate (“metal soap”) is used pure ordissolved and wherein said liquid is then formed into droplets andoxidized in a high temperature environment, in particular a flame.

The current invention uses metal carboxylates (salts of metals with oneor several carboxylic acids) as a metal source for high temperatureconversion to nanoparticle oxides, in particular flame spray synthesis.The metal soaps are used pure or dissolved in an additional solvent asto achieve a suitable viscosity.

In a much preferred embodiment of the inventive method the precursorliquid essentially consists of one or several metal soaps (presence ofusual impurities of technical solvents is acceptable) pure or dissolvedin a solvent. Optionally, these precursors may be heated prior tospraying. The liquids are characterized by a net heat of combustion ofat least 13 kJ/g for some metals, preferably, and more generallyapplicable at least 18 kJ/g, more preferably at least 22.5 kJ/g, mostpreferably at least 25.5 kJ/g, and a viscosity of less than 100 mPas,preferably less than 50 mPas, most preferably less than 20 mPas. This isachieved by using pure metal soaps (optionally heated to elevatedtemperature that reduces their viscosity) or by dissolving metal soapsin an additional solvent with suitable viscosity and combustion energy.

In order to reduce the viscosity of the precursors it is often favorableto use a mix of a hydrocarbon such as toluene, xylene, hexane or lightparaffin oil and a metal carboxylate where the corresponding carboxylicacids have a mean carbon number of at least 3 per carboxylate group asto ensure low viscosity, sufficient solubility and combustion energy.Preferred solvent mixes comprise metal soaps with a mean carbon numberper carboxylate group of at least 4, more preferred of at least 5, inparticular 5 to 8, whereby the carboxylic acid usually has not more than30 C and preferably is selected from one or more C3 to C18monocarboxylic acids, more preferred from one or more C4 to C12carboxylic acids, and most preferred from one or more C5 to C8carboxylic acids. Even though a higher mean carbon content than 8 can beused, such higher mean carbon content usually reduces the productionrate.

Also suitable are chelating acids, such as dicarboxylic acids,polycarboxylic acids, amino carboxylic acids, hydroxy carboxylic acids,provided that they provide sufficient enthalpy or are compatible withthe solvent optionally present to lower the viscosity and/or to enhancethe enthalpy. Suitable carboxylic acids comprise linear or branchedchain acids that can be saturated or unsaturated, and optionally furthersubstituted as long as the substituents do not unduly affect the highenthalpy of the acid that preferably is at least 13 kJ/g, morepreferably at least 18 kJ/g, much preferably at least 22.5 kJ/g and mostpreferably at least 25.5 kJ/g, or the melting point or the solubility ofthe metal soap. For many heavy metal oxides, metal soaps of2-ethylhexanoic acid exhibit the desired properties and are well suitedfor flame spray synthesis. In other cases, such as the production oftitanates, the metal soaps can be directly made in situ from a metalcontaining precursor by reaction with a carboxylic acid. Such solutionscan be mixed with another metal soap and readily produce metal oxides,e.g. titanates such as calcium- and barium titanate. As alreadymentioned above, suitable metal precursors are in particular those withan anion that in protonated form can easily be removed, e.g. by heating,optionally under vacuum.

Beyond metal soaps with unsubstituted monocarboxylic acids, as alreadyaddressed above, other metal carboxylic acid salts can be applied as faras some limitations are observed. In this case it may be advantageous oreven necessary to add an acid to the solvent in an amount of usually atmost 40%. Dependent of the one or more acids derived anions of the metalsoap, the one or more acid used as solvent can be linear or branched,saturated or unsaturated, unsubstituted or substituted monocarboxylicacids provided that they result in a suitable high enthalpy solvent.Such acids comprise acids with polar substituents such as —OH, —NH₂ or—CONH₂ groups that can be used to adjust the solvent to specific needs.In specific cases also sufficiently long chain optionally substitutedsaturated or unsaturated dicarboxylic acids or polycarboxylic acids canbe used.

In many cases, however, no acid is needed or even disadvantageous. Ithas been found that metal carboxylates with higher mean carbon content,in particular salts with at least C4 monocarboxylic acids, preferably atleast C5 monocarboxylic acids, are readily dissolved in apolar solventswith low molecular weight and/or low viscosity such as toluene, xylene,lower alkanes such as hexane, white spirits, but also light paraffinoil, ethers etc.

Presently preferred metal soaps are those with unsubstituted, linear orbranched, saturated or unsaturated monocarboxylic acids, in particularC5 to C8 monocarboxylic acids such as e.g. 2-ethyl hexanoic acid, andpreferred solvents are at least 60%, preferably at least 80%, muchpreferred almost 100% apolar solvents, in particular solvents selectedfrom the group comprising toluene, xylene, lower or low viscosityalkanes, such as hexane, isooctane, lower or low viscosity alkenes,lower or low viscosity alkines, or mixtures thereof.

In yet another manifestation of the present invention, an oxide can beconverted into nanoparticles by in situ forming the metal soap bytreating the precursor, e.g. the oxide, hydroxide, halide, carbonate oralkoxide, at elevated temperature with the corresponding one or morecarboxylic acids, eventually reducing viscosity by adding an additionalsolvent and oxidizing, in particular flame spraying the resultingmixture. The resulting oxide is made of nanoparticles of very narrowsize distribution. Scheme 1 below outlines the procedure.

Scheme 1. The carboxy process for the conversion of an oxide intocorresponding nanoparticles.

Procedure Example Oxide Gadolinium oxide (99%) Make metalic soap 66.6 gGd2O3, 200 ml of 2- Ethylhexanoic acid, 35 ml acetic acid anhydride,reflux at 140° C. for 10 h, some acetic acid removed Adjust viscosityDilute metal soap with xylene Flame spray Spray in a methane/oxygenflame as reported in the Experimental section Collect Gd2O3nanoparticles with nanoparticles 70 m2/g surface area.

In the inventive method, the flame or high temperature zone has atemperature of at least 600° C., usually at least 1200° C., preferablyat least about 1600° C. A preferred range of the flame temperature formany applications is 1200 to 2600° C.

The average diameter of the droplets can vary depending on the liquiddispersion setup and the properties of the liquid itself. Usually, theaverage droplet diameter ranges from 0.1 μm to 100 μm, preferably from 1μm to 20 μm. The droplet diameter is of minor importance if a heredescribed precursor solution is applied. This is very advantageous as itallows for most conventional, commercially available oil-burners toconvert the here described liquid into corresponding oxides. Suitableoil-burners are—to only mention a few—available from Vescal AG,Heizsysteme, Industriestrasse 461, CH-4703 Kestenholz under thedesignation of OEN-151LEV, or OEN-143LEV, or OEN-331LZ to OEN-334LZ.

Preferably, the droplets subjected to heat oxidation comprise the metalin concentrations of at least 0.15 moles metal per liter. Suchconcentrations lead to production rates of at least 0.15 moles metal ormetal oxide per nozzle.

If the method of this invention is performed using a flame withinsufficient oxygen for full conversion, such oxygen defficiency resultsin the formation of substoichiometric oxides or metals and mixturesthereof that have also their applications.

Furthermore, by adding a further step after the oxidation, the asproduced metal oxides can be converted to the corresponding non-oxidessuch as nitrides, borides, carbides by means of an additional treatment,such as a treatment with ammonia, hydrogen, etc.

The method of the present invention can be used for the production of abroad range of metal oxides, in particular also mixed metal oxides.Further oxide systems that may be prepared by the inventive method,using specific carboxylic acids as solvent are e.g. pure transitionmetal oxides and mixed alkali or alkaline earth metal and transitionmetal oxides such as alkali metal manganates, especially lithiummanganate or cobaltates, or calcium, strontium, barium titanates orzirconates and mixtures thereof, but also other oxides such as iron ormanganese oxides.

Such oxides, in particular if prepared from pure precursors, have a highthermal stability that is characterized by a specific surface area (BET)of at least 20 m²/g after sintering for 1 h at 600° C.

In contrast to impurity free pure of mixed metal oxides, oxides withimpurites and superior features than hitherto known can be produced thatusually have a specific surface area (BET) of at least 5 m²/g after hightemperature oxidation, preferably at least 10 m²/g, in particular atleast 15 m²/g. Impurities allowing such surface areas are e.g. alkalimetals, earth alkali metals, transition metals or rare earth metals, butalso chlorides, fluorides or bromides, phosphates, sulfates or siliconand main group metals, such as Al, B etc., in the range of 0.5 to 5% perweight, in particular from 0.8 to 5% per weight.

A preferred metal oxide obtainable by the method of the presentinvention is characterized by a geometric standard deviation of the massaveraged primary particle size distribution smaller than 2 or with lessthan 2 wt % of solid primary particles with more than 500 nm in diameterand a specific surface area of more than 3 m²/g. Such metal oxideparticles are suitable for powder injection molding or optical lenspolishing.

Much preferred metal oxides are zirconia stabilized with cerium and/oryttrium, preferably cerium or yttrium, in particular in an amount ofcerium and/or yttrium of at most 20%.

Ceria based oxides in general and zirconia based oxides in general, butalso LiNbO₃, BaTiO₃, SrTiO₃, CaTiO₃, Li_(y)MnO_(x) and derivatives, NiO,Li_(x)TiO_(y), apatite for medical implants, metal doped titania, rareearth metal oxides, especially lanthanum based perowskites, mixed oxidescontaining an element of the earth metal and from the transition metalgroup, mixed oxides containing an element from the alkali metals and thetransition metals, aluminates, borates, silicates, phosphates, hafnia,thoria, uranium oxide, etc. with specific properties are obtainable. Aseries of representative examples is given in the experimental section.

The method of the present invention also encompasses the production ofmetal oxides starting from metal soaps that are combined with otherprecursors, in particular organometallic or organometalloide precursors,provided that the solubility and enthalpy stay within the hereinoutlined ranges. The as-prepared oxides may be subjected directly to anafter-treatment in order to form nitrides, carbides, silicides, boridesand others.

Suitable metals for the production of pure or mixed metal oxides are e.gcerium, zirconium, any rare earth metals, lithium, sodium, potassium,rubidium or caesium, magnesium, calcium, strontium, barium, aluminium,boron, gallium, indium, tin, lead, antimony, bismuth, scandium, yttrium,lanthanum, titanium, hafnium, vanadium, niobium, tantalum, chromium,molybdenum, tungsten, mangenese, rhenium, iron, ruthenium, osmium,cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver,gold, zinc, cadmium, thorium, uranium or silicon, whereby preferredmixed metal oxides are cerium with a rare earth or zirconium and/oralumina, zirconium with yttrium, scandium, aluminum or an alkali earthmetal, titanium and an alkali or alkali earth metal, manganese, cobalt,nickel and iron in combination with lithium or another alkali metal,lithium and niobium, tungsten or molybdenum, barium with aluminium andplatinum, aluminium with platinum or palladium, copper and aluminium orzirconium and zinc, lead and an alkali or earth alkali metal, tin andplatinum, indium and tin or zinc, lanthanum and iron, manganese, cobaltor nickel, magnesium and aluminum.

A lot of cheap more or less pure metal chlorides are commerciallyavailable. Therefore, it is an advantage of this invention that amixture of water free metal chlorides can be used as a metals source.Said mixture is then reacted with a carboxylic acid, and thereby formedhydrogen chloride is removed by degassing and/or heating the resultingsolution. Such solution then can be used for the manufacture of oxideswith a chloride content of less than 0.1% by weight.

The metal oxides of the present invention, in particular the mixed metaloxides, have the applications already known for them and an extendedfield of applications due to their improved properties.

Metal oxides of the present invention, such as e.g. ceria, zirconia orceria/zirconia, due to their great homogeneity can be used forchemomechanical polishing, provided that they are of high purity.

Given the homogeneous particle structure, the oxides of the presentinvention are furthermore suitable for the manufacture of coatings, instructural ceramics or for protecting layers on metals.

The metal oxides of the present invention are in general e.g. usable asat least part of a catalytically active system, in particular forcombustion engines, and/or for chemomechanical polishing, and/or aselectrolyte or membranes in solid oxide fuel cells, and/or in batteriesor in rechargable batteries, especially in Li ion batteries, and/or forat least one of the following purposes: as heterogenous catalysts, asNOx storage catalysts, as lubricant, as structural ceramics, as batterystorage materials, for chemical sensors, for elements in energyproduction, for solar energy production elements, for electron storagein recyclable battery units, as dielectrics, as piezoelectrics, inmicro-actuators, as ferroelectric, as gas permeable membranes, aspigments, polymer additives, stabilizers; magnetic fluids, polishingpowders, additives in metal alloys, in armor fabrication, inmicroelectronics, as electrode raw material, as phosphors for radiationsensitive elements and in displays, in lasers, cosmetics, pharmaceuticalpackaging, additive in food and pharmaceutical applications, fuel cells,and/or superconductors.

One preferred use of ceria, and/or preferably ceria/zirconia having amonolithic structure as obtainable according to the present invention isas a catalyst. For such catalyst, the ceria and/or the ceria/zirconiacan be mixed with monolithic structure giving material such as Al₂O₃.The ceria and/or ceria/zirconia can e.g. be a layer covering amonolithic structure carrier.

Such catalyst may furthermore comprise additional catalytically activesubstances such as further metal oxides, •e.g. titania, vanadia,chromia, manganese, iron, cobalt, nickel, copper oxides, and/or noblemetals, e.g. platinum, palladium, rhodium, ruthenium, rhenium, iridium,all of them alone or in admixture with one or more thereof, or alloysthereof. A preferred catalyst is platinum/ceria/zirconia.

EXAMPLES

General Procedure

Mixed oxide powders were produced by flame spray pyrolysis in alaboratory scale setup [Mädler et al. (2002A), FIG. 1] or a pilot-scaleoil-burner (SYSTHERM, CH-8105 Regensdorf, Typ NS1, Ausführung 1.1, 1988,Nr. 20940; Control unit (Satronic Type ZT801 and MMD 870; Flickerdetector (Satronic IRP 1010); Burner tube length: 0.23 m, diameter:0.0825 m; 720 l of air/hour as dispersion gas, 3 liter of liquid perhour, FIGS. 1 and 2).§ The following describes a standard preparationlater denoted as (3/3). Metal containing liquids are brought into theflame by a syringe pump (Inotech IR-232) at 3 ml/min. The flame consistsof a central spray delivery, a premixed, circular support flame(diameter 6 mm, slit width 150 μm) and a circular sheet gas delivery(ring of sinter metal, inner diameter 11 mm, outer diameter 18 mm, 5 loxigen/min.). Oxygen (Pan Gas, 99.8%) was used as a dispersion gas inall experiments and delivered at 3 l/min. A mixture of methane (1.5l/min, Pan Gas, 99%) and oxygen (3.2 l/min) was fed to the inner slitand formed a premixed flame. All gas flow rates were controlled bycalibrated mass flow controller (Bronkhorst EL-Flow F201).

Preparation of Metal Containing Precursors

Corresponding amounts of metal soaps are dissolve in xylene, toluene,petroleum, light paraffin oil or other suitable solvents. Optionally,some carboxylic acid is added. The following gives a series ofpreparations with specific substances.

Production of Iron or Manganese Oxide Nanoparticles

Iron oxide. 10 ml of iron naphthenate (Strem Chemicals, CAS no.[1338-14-3], LOT no. 138222-S, 80% in mineral spirits, 12 wt % iron) aremixed with 10 ml of xylene resulting in a dark red solution. Sprayingthis precursor at 3 ml/min (same experimental parameters as in theceria/zirconia experiments) in a methane/oxygen spray flame results ironoxide nanoparticles with a BET specific surface area of 71 m²/g.Production rate: 85 g iron oxide/liter precursor liquid.

Manganese oxide. 10 ml of manganese naphthenate (Strem Chemicals, CASno. [1336-93-2], LOT no. 124623-S, 56% in mineral spirits, 6.0 wt % Mn)are mixed with 10 ml of xylene and flame sprayed at the same conditionsas for the iron oxide. The corresponding specific surface area yields 80m²/g and X-ray diffraction confirms the formation of Mn3O4 as a majorcomponent. Production rate: 42 g manganese oxide/liter precursor liquid.

Lithium manganese spinell. A solution of 0.75 M manganese naphthenateand 0.375 M lithium octoate is sprayed at (3/3) in a oxygen/methaneflame. The product is kept at 400° C., in air for 1 h. XRD confirms theformation of the spinell phase with a specific surface area of 91 m2/g.Narrow particle size distribution is confirmed by TEM (see FIG. 9).

Calcium titanate. 20 ml of a 1.24 M Ca octoate solution (in whitespirit/2-ethylhexanoic acid) and 7.4 ml of titanium tetra isopropoxideare mixed. The solution warms up as the isopropanol is replaced by thestronger acid group. The solution is then diluted by two volumes ofxylene and sprayed at (3/3) resulting a white powder. XRD confirmsformation of calcium titanate (see FIG. 8) with a specific surface areaof 60 m²/g. Calcination at 600° C., 1 h, air results in 45 m²/g.

Barium titanate. 10 ml of a 0.5 M barium octoate solution(2-ethylhexanoic acid/toluene) and 1.51 ml of titanium are mixed. Aftercooling down, 5 ml of toluene are added to reduce the viscosity.Spraying at (3/3) results a pure white powder. Raw material is ratheramorphous, but keeping it at 600° C., 1 h in air results a pure bariumtitaniate phase (XRD) with specific surface area of 36 m²/g.

Production of Calcia Stabilized Zirconia

Laboratory scale. 3.5 ml of zirconium (IV) octoate (technical grade,SocTech SA, Bucharest, Rumania, CAS no. [18312-04-4], 16 wt %zirconium), 8.5 ml of 2-ethylhexanoic acid, 5.5 ml of toluene and 0.5 mlof calcium octoate (technical grade, SocTech SA, Bucharest, Rumania, CASnr [242-197-8], 5.2 wt % calcium) are mixed and sprayed using thestandard parameters. The as-prepared powder has a specific surface areaof 62 m²/g, sintering in air (16 h at 700° C., ramp at 5° C./min)reduces it to 19 m²/g.

Pilot scale (first, not optimized trial). Spraying a mixture of 2 kgzirconium (IV) octoate (technical grade, SocTech SA, Bucharest, Rumania,CAS no. [18312-04-4], 16 wt % zirconium), 0.66 liter light paraffin oiland 0.225. liter of calcium octoate (technical grade, SocTech SA,Bucharest, Rumania, CAS no. [242-197-8], 5.2 wt % calcium) at 3 kg/h ina larger burner (see General Procedure) results a white, homogeneouspowder with 24.5 m²/g. Production rate: 180 g/liter precursor.

Production of Ceria and Ceria/Zirconia

Laboratory scale. For ceria, 10.5 ml of cerium (III) octoate (SocTechSA, Bucharest, Rumania, technical grade, CAS no. [56797-01-4], 10.2 wt %cerium, contains 0.14 wt % Na) are diluted with 5.5 ml toluene and 2 ml2-ethylhexanoic acid. Spraying using the standard parameters results in67 m²/g and 10 m²/g after sintering in air (16 h at 700° C., ramp at 5°C./min). Spraying high grade precursors results in much higher stabilityas discussed in the Results part.

Pilot scale (first, not optimized trial). Spraying a mixture of 0.75 kgcerium (III) octoate (SocTech SA, Bucharest, Rumania, technical grade,CAS nr [56797-01-4], 10.2 wt % cerium, contains 0.14 wt % Na) and 0.275liter light paraffin oil at 3 kg/h in a larger burner (see GeneralProcedure) results a slightly yellowish, homogeneous powder with 18 m²/gafter sintering in air (16 h at 700° C., ramp at 5° C./min) 4 m²/g.Production rate: 90 g/liter precursor.

Laboratory scale. For ceria/zirconia 10.5 ml of cerium (III) octoate(SocTech SA, Bucharest, Rumania, technical grade, CAS no. [56797-01-4],10.2 wt % cerium, contains 0.14 wt % Na) and 3.5 ml of zirconium (IV)octoate (SocTech SA, Bucharest, Rumania, CAS no. [18312-04-4], 16 wt %zirconium) are diluted with 4 ml toluene. Spraying using the standardparameters results in 69 m²/g and 28 m²/g after sintering in air (16 hat 700° C., ramp at 5° C./min). Even using this technical qualityprecursors, the effect of stabilization is clearly visible.

Pilot scale (first, not optimized trial). Spraying a mixture of 2.5 kgof cerium (III) octoate (SocTech SA, Bucharest, Rumania, technicalgrade, CAS no. [56797-01-4], 10.2 wt % cerium, contains 0.14 wt % Na),0.833 kg zirconium (IV) octoate (SocTech SA, Bucharest, Rumania, CAS no.[18312-04-4], 16 wt % zirconium) and 0.833 l toluene at 3 kg/h in alarger burner (see General Procedure) results a slightly yellowish,homogeneous powder with 21 m²/g after sintering in air (16 h at 700° C.,ramp at 5° C./min) 14 m²/g. Production rate: 125 g/liter precursor.

Results and Discussion

The Importance of Viscosity

As the metal carrier liquid is dispersed into small droplets duringspraying, viscosity has to be sufficiently low as to allow good liquiddistribution. The following Table 1 illustrates in what range the liquidviscosity is appropriate for flame spray synthesis.

TABLE 1 Suitable for Viscosity/ flame spray Metal carrier liquid mPa sif at 298 K Iron naphthanate 80% in white >100 no spirit Ditto, dilutedwith xylene (1:1) 3 yes Manganese naphthenate 56% in 16 yes white spiritDitto, diluted with xylene (1:1) 1.7 yes 0.4 M cerium, 0.4 M zirconiumin 5 yes 2-Ethylhexanoic acid/Toluene (5:2 by volume) Zirconium octoate(16 wt % Zr) in >100 no white spirit Ditto, diluted with xylene (3:1) 10yes Ditto, diluted with dodecane 19 yes (3:1) Cerium octoate (10.2 wt %cerium) 22 yes in white spirit, diluted with xylene (3:1) Ditto, dilutedwith dodecane >40 dependent on (3:1) burner Note: Viscosity measured ina rheometer (Haake VT 550 Rheometer, Fisons, Digitana AG, 8810 Horgen)at ambient temperature

Diluting metal carboxylates with other solvents shows the colligativeproperties of these solutions. Xylene reduces the viscosity about twiceas good as dodecane for both cerium and zirconium precursors.

As an alternative to dissolution, or in addition thereto, heating of themetal carboxylate or the metal carboxylate comprising mixture/solutioncan be provided, in order to further lower the viscosity to preferablybeyond 40 mPa s.

The Importance of Material Purity on Thermal Stability

As additional elements, sodium or chloride form the manufacturingprocess of the carboxylate salts of metals, have a significant influenceon the final product stability, a comparison is given form precursors ofhigh purity (>99% metal content) and technical grades (below 99% metal,contains chloride and mainly sodium). A large drop in stability aftersintering becomes most apparent for ceria where sodium in the technicalprecursor increases its sintering rate. A cross experiment with purecerium octoate and an additional 1 wt % sodium resulted in a similarloss in specific surface area as in the case of the technical gradeprecursors. The results are shown in Table 2 below.

TABLE 2 SSA as Prod. prepared SSA sintered Rate grade [m² g⁻¹] [m² g⁻¹][g l⁻¹] Ceria 99.8%   125 71 69 ″ Tech. 67 10 69 Ceria, 1 wt % —  24^(a)— Na Zirconia >99% 105 45 50 ″ Tech. 121 24 50 Ceria/zirconia >99% 94 8359 ″ Tech. 69 28 59 Note: Technical grades contain sodium and chlorideamongst others. ^(a)Pure, flame made ceria is impregnated with sodiumhydroxide (1 wt % Na₂O in final ceramic) and subjected to sintering.grade: purity based on metals only

The Range of Accessible Materials

The given examples show that single transition metal oxides can beprepared with above 60 m²/g specific surface area while maintainingproduction rates of more than 40 g/h. Such materials find applicationsin a wide range of products and processes. Electronic materials,batteries, ferroelectrica, permanent magnets, coils, and magnetic fluidsare just a few.

The given examples further show that an alcaline or earth alcaline metalcan be combined with a transition metal oxide to form the correspondingmixed oxides. Such materials can form spinells, perovskites and otherinteresting phases. They find applications as dielectric, piezoelectics,actuators, in membranes, as sensors, in capacitors, superconductors andothers. Some are used as catalysts, as ceramics for high temperatureapplications or structural ceramics.

Material Homogeneity

Transmission electron microscopy images of lithium manganate, calciumtitanate or ceria/zirconia show that nanoparticle with a narrow range ofparticle sizes can be obtained by the here described method. This isfurther supported by X-ray diffraction pattern that would show theformation of large crystallites. Specific surface area, furthermoresupports the particle size range as observed by TEM. This data clearlyshow that the present invention can be used for the manufacture ofnanoparticles.

CONCLUSIONS

The carboxy process offers a readily accessible way to nanoparticleproduction. Using metal soaps as main precursors even conventional oilburners may be used to synthesize nanoparticles. Particles of transitionmetal oxides, mixed oxides from elements from the alkali metal, alkalineearth metal, rare earth metal and transition metal series amongst themhave been made showing the versatility of the process. Handling, storageand mixing compatibility of such metal soap based liquids are combinedwith high production rate. Enabling over 200 g of nanoparticles to beproduced from 1 liter of precursor liquids while being ratherinsensitive to the type of burner, dispersion gas system or flame typemakes these precursors very advantageous over any other metal deliverysystem for flame spray synthesis.

Even technical precursors may be used but consequently result in muchlower thermal stability. A proof of this effect was made by dopingsodium into a pure ceria precursor (see Table 2).

While there are shown and described presently preferred embodiments ofthe invention, it is to be distinctly understood that the invention isnot limited thereto but may be otherwise variously embodied andpracticed within the scope of the following claims.

REFERENCES

-   Laine, R. M., Hinklin, T., Williams, G., Rand, S. C.; Low Cost    Nanopowders for Phosphor and Laser Applications by Flame Spray    Pyrolysis, J. Metastable Nanocryst. Mat., 2000, 8, 500-   Aruna, S. T., Patil, K. C.; Combustion Synthesis and Properties of    Nanostructured Ceria-Zirconia Solid Solutions, NanoStructured    Materials, 1998, 10, 955-   Laine, R. M., Baranwal, R., Hinklin, T., Treadwell, D., Sutorik, A.,    Bickmore, C., Waldner, K., Neo, S. S.; Making nanosized oxide    powders from precursors by flame spray pyrolysis, Key. Eng. Mat.,    1999, 159, 17-   Yoshioka, T., Dosaka, K., Sato, T., Okuwaki, A., Tanno, S., Miura,    T.; Preparation of spherical ceria-doped tetragonal zirconia by the    spray pyrolysis method, J. Mater. Sci. Lett., 1992, 11, 51.-   Oljaca, M., Xing, Y., Lovelace, C., Shanmugham, S., Hunt, A., Flame    Synthesis of nanopowders via combustion chemical vapor    deposition, J. Mater. Sci. Lett., 21, 621-626 (2002).-   Maric, R., Seward, S., Faguy, P. W., Oljaca, M., Electrolyte    Materials for Intermediate Temperature Fuel Cells Produced via    Combustion Chemical Vapor Condensation, Electrochemical and Solid    State Letters, 6 (5) A 91 (2003).-   Mädler, L., Kammler, H. K., Mueller, R., S. E. Pratsinis; Controlled    synthesis of nanostructured particles by flame spray pyrolysis,    Aerosol Science, 2002A, 33, 369-   Mädler, L., Stark, W. J., Pratsins, S. E., Flame-made Ceria    Nanoparticles, J. Mater. Res., 2002B, 17, 1356.-   W. J. Stark, L. Mädler, M. Maciejewski, S. E. Pratsinis, A. Baiker,    Flame-Synthesis of Nanocrystalline Ceria-Zirconia: Effect of Carrier    Liquid, Chem. Comm., 2003, 588-589.

1. A method for manufacturing a metal oxide or a mixed metal oxide, saidoxide being in the form of nanoparticles, wherein at least one metalprecursor is contained in a precursor solution which is formed intodroplets and the at least one metal precursor is oxidized in a flame,characterized in that a) said at least one metal precursor is a metal2-ethyl hexanoate; b) said precursor solution, prior to being formedinto droplets, has a viscosity of at most 100 mPas; c) said precursorsolution is i) heated to obtain such viscosity or ii) mixed with atleast one viscosity reducing solvent, said solvent is at least 60%apolar; d) said flame oxidation is performed in a spray burner thatcomprises nozzles; e) said precursor solution comprises at least 0.15moles metal per liter of the precursor solution; and f) at least 0.15moles of a metal oxide or a mixed metal oxide per nozzle and per literof the precursor solution are produced, wherein the metal or metals inthe metal oxide or the mixed metal oxide is/are selected from the groupconsisting of zirconium, lithium, sodium, potassium, rubidium, cesium,magnesium, calcium, strontium, barium, aluminum, boron, gallium, indium,tin, lead, antimony, bismuth, titanium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, tungsten, mangenese, rhenium, iron,ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium,platinum, copper, silver, gold, zinc, cadmium, thorium, uranium andsilicon.
 2. The method of claim 1, wherein the solvent is free of acid.3. The method of claim 1, wherein the solvent comprises at least one lowmolecular weight and/or low viscosity apolar solvent selected from thegroup consisting of toluene, xylene, lower aliphatic hydrocarbons andmixtures thereof.
 4. The method of claim 1, wherein the mixed metaloxide contains a combination of metals selected from the groupconsisting of zirconium in combination with aluminum or an alkali earthmetal; titanium in combination with an alkali or alkali earth metal;manganese, cobalt, nickel and iron in combination with an alkali metal;lithium in combination with niobium, tungsten or molybdenum; barium incombination with aluminum and platinum; aluminum in combination withplatinum or palladium; copper in combination with aluminum or zirconiumand zinc; lead in combination with an alkali or alkali earth metal; tinin combination with platinum; indium in combination with tin or zinc;and magnesium in combination with aluminum.
 5. The method of claim 1wherein the oxidation is performed at a temperature of at least 600° C.6. The method of claim 1 wherein the metal precursor is preparedstarting from a metal oxide, a metal hydroxide, a metal carbonate, ametal halide or a metal lower alkyl oxide.
 7. The method of claim 1,wherein a net heat of combustion of the metal precursor is at least 13kJ/g.
 8. The method of claim 1, wherein the metal precursor compriseimpurities of one or more elements of the group comprising alkalimetals, alkaline earth metals, transition metals, rare earth metals,chlorides, fluorides, bromides, phosphates, sulfates, silicon, and maingroup metals, whereby the impurites are present in amounts in the rangeof 0.5 to 5% by weight.
 9. The method of claim 1, wherein the metaloxide is a sub-stoichiometric oxide and wherein said metal oxide isproduced in a flame with insufficient oxygen for full conversion.
 10. Amethod of claim 1 wherein one or more water free metal chlorides areused as a metals source, said metals source is reacted with a 2-ethylhexanoic acid such that hydrogen chloride is generated and issubsequently removed, whereby the precursor solution containing at leastone metal precursor is obtained that is suitable for the manufacture ofoxides with a chloride content of less than 0.1% by weight.
 11. Themethod of claim 1 wherein said viscosity reducing solvent is almost 100%apolar.
 12. The method of claim 1 wherein said viscosity reducingsolvent is selected from the group consisting of toluene, xylene,hexane, isooctane, or mixtures thereof.
 13. The method of claim 1wherein said metal precursor is heated to obtain said viscosity.
 14. Themethod of claim 1 wherein the metal or the combination of metals in saidat least one metal precursor is selected from the group consisting ofzirconium, titanium, iron, manganese, lithium, calcium, barium andmixtures thereof.
 15. The method of claim 1 wherein the metal in said atleast one metal precursor is zirconium.